ISOTOPIC EVIDENCE FOR THE MULTIPLE SOURCES OF LEAD IN PORPHYRY CU-MO AND BASE METAL LODE ORES AT BUTTE, MONTANA

Porphyry Cu-Mo and Main Stage Cu-Zn-Pb-Ag-Mn lodes are associated with porphyry intrusions and hydrothermal activity spanning 3 to 4 m.y. (~66 to 62 Ma) and post-date host Butte quartz monzonite (BQM, 76 Ma).  The total amounts of Cu and Pb deposited are ~35 and 0.5 Mt, respectively, with 70% of Cu but little Pb deposited in porphyry stage and 30% of Cu and almost all Pb deposited in the younger lode stage. Porphyry Cu-Mo fluid inclusions (~3 to 5 wt.% salts), trapped in the one-phase field at high temperature, represent magmatic aqueous fluid samples with ~900 to 10,000 ppm Cu and ~20 to 60 ppm Pb (Rusk et al., 2004); the Cu/Pb ratio of ~100 is similar to the total Cu/Pb ratio of the Butte ores.  Another source of Pb in <450°C fluids characteristic of lode mineralization is via destruction of Pb-bearing K-feldspar during wall-rock alteration;  this process likely adds Pb to hydrothermal fluids and potentially to lode ores.

To test these alternate hypotheses, multi-collector, LA-ICP-MS analysis of Pb isotopes was conducted via in situ laser ablation of K-feldspar and sulfides.  The Pb isotopic ratios illustrate distinct fields for BQM (99% of host rock), mineralized quartz porphyry, and late rhyodacite porphyry. The Pb isotopic composition in hydrothermal K-feldspar in BQM formed as selvages on porphyry Cu-Mo veins indicates that little of this Pb is derived from BQM and that most is derived from mineralizing porphyries possibly mixed with Pb derived from an older less radiogenic source rock.  In contrast, sulfides from Main Stage lodes have ²⁰⁸Pb/²⁰⁶Pb ratios (2.13 to 2.14; error <0.2%) similar to and lower than mineralizing porphyries (2.135-2.145) and may contain a component of Pb derived from late rhyodacite dikes (2.125-2.129) or BQM. These isotopic data are consistent with magmatic-derived Pb in the porphyry Cu-Mo stages, but both magmatic and wall-rock alteration sources of Pb in the lode systems.